Take and FX PHD class. They have some classes on compositing and matte painting from time to time. They can get you some decent source footage that you can play with and take you the correct direction.

you need to learn first:
how works perspective/volumes
how works lights&shadows
how works texture&materials
how to quick conceptualize your idea into a clear picture
how to transform your concept into a digital mattepainting
how to do a 3d projection & cam movement
how to composite your layers
how to composite and achieve your vfx shot

i fount those courses not very well, too expensive and not really teaching you reals tricks used in movie industry, you can find TONS of tutorials like those, but your'e never going to find advanced tricks on HOW to achieve a PRO mattepainting quality work.
Because people in the vfx industry don't teach those "secret" tricks....

Do you have access to a camera and a tripod? Even an iphone will do for the sake of learning.

Just head out and shoot some video and do simple background replacements, extensions etc

Shoot some footage, open the video file in Photoshop or After Effects, Find a nice frame to work over, then begin adding in desired elements. The elements can be anything, just pull off some high res images from google. The practice will be more about how you integrate the elements into the frame, making correct use of perspective, color correction, matching black values, correct lighting, clean extraction, correct depth of field etc

One note; if you extract elements, for example a ruined car, and intend to add it into a landscape. Be mindful of local distortion. This is the distortion in the element itself (ruined car) you will need to undistort every element and correctly match the lens distortion of the target frame. Does this make sense? Basically if there was a ruined car in your frame, then it would have the distortion effect of the frame. But when you find refs from google etc, they always have there own local distortion due to different lens, so you have to correct this before extracting.

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